Euro Cylinder Protrusion: Testing the Three-Millimetre Rule Against a Wrench Torque Model

Euro-profile cylinders that stand proud of the door furniture can be gripped and snapped, and fitting guidance commonly allows at most 3 mm of protrusion. This study tests that rule against a stated torque model in which a wrench attack develops T = 18 + 2.6 p newton-metres at protrusion p millimetres, a standard cylinder bridge fails at 28 Nm, and a reinforced cylinder with a sacrificial cut fails at 55 Nm. The standard cylinder's margin is exhausted at a protrusion of 3.9 mm: at the customary 3 mm the attack torque is 25.8 Nm, already close to the bridge strength, while at 10 mm it reaches 44.0 Nm and the cylinder fails with room to spare. The reinforced cylinder holds its margin until 14.3 mm of protrusion. The 3 mm rule is therefore confirmed as a sensible fitting limit for standard cylinders, but it is a limit with little slack, and the calculation argues for measuring protrusion at fitting and for choosing a reinforced cylinder whenever the door and furniture combination cannot be brought under about 3 mm. SIMULATED, NOT MEASURED.